The toxicity of various concentrations of oxalic acid dihydrate (OA) in aqueous and
sucrose solution to Varroa destructor and to honey bees (Apis
mellifera) was assessed using submersion tests of caged bees and by spraying
bees in colonies with and without brood. An aqueous solution of 0.5% OA gave effective
control of the mite and was non-toxic to bees whereas higher concentrations of OA
(1.0–2.0%) were highly toxic to bees. Submersion tests into solutions with 0.1% OA were
acaricidal both in aqueous (59.9 ± 3.7 %) and in 50% sucrose solution (71.1 ± 4.2%)
whereas concentrations of 0.2–0.5% OA were highly effective; OA in sucrose solution was
more toxic to bees than OA in the aqueous solution. Spraying with 0.5% OA solution at a
dose of 25 mL per comb in May 2003 and in April 2004 was 99.01–99.42% effective in mite
control in Estonian standard one box long beehives with 22 frames (each 414 × 277 mm, area
1000 cm2 per comb side). Most mites fell after the first spraying. In autumn,
spraying test colonies that had little capped brood once or twice with a 0.5% OA solution
gave effective mite control (92.94 ± 0.01% and 91.84 ± 0.02%, respectively) with no
noticeable toxicity to bees.